The distance from a radiation source at which a radiation beam is focused may be controlled by movement of one or more focusing lenses along the beam axis so as to change the focal point along the beam axis. With such prior art systems one or more lenses which are of relatively large mass must be moved. As is well understood, the larger the mass, the more difficult it is to rapidly accelerate the same. Consequently, the rate at which focusing may be changed with such prior art movable lens systems is limited by the mass of the lens and associated mounting mechanism to be moved. Furthermore, if the lens is not moved precisely along the beam axis, the beam will be deflected laterally at the movable lens thereby imparting undesired movement to the focal point. Control of movement of the lens, or lenses, precisely along the beam axis to prevent such unwanted beam deflection is difficult and expensive.
The focusing control system of the present invention is well adapted for use in a laser beam scanning system such as that used in the exposure of printing plates, the inspection of printed circuit boards, or the like. For example, it is useful in a scanning system that employs pre-deflection optics, i.e. a system wherein the laser beam passes through a focusing lens before it is deflected as by use of a rotating polygon, oscillating mirror, or the like. In scanning systems using pre-deflection optics, a very simple inexpensive lens is adequate to produce a diffraction-limited focused spot. However, with such systems the locus of the focused spot is not a straight line but is, in general, a curved line. If, for example, the deflector is an oscillating mirror whose axis of rotation is in the plane of the mirror, the locus of the focused spot is an arc of a circle. Generally, the desired focus is a straight line in which case the laser beam will be out of focus on the straight line except at one or two points where the arc is tangent to or intersects the line.